Heretofore, aviator's night vision imaging goggles have been designed to operate at light levels down to overcast starlight conditions. However, conditions can exist where the minimum light required is not available. Presently in use are filtered incandescent lamps to provide IR illumination to compensate for the absence of natural light when operating under extremely low light conditions.
While IR filtered incandescent lamps provide more than adequate IR illumination they have three major drawbacks:
1. The filter traps significant amounts of heat which reduces the filament life of the lamp to a fraction of its normal life. Indications are that the lamps used in standard landing systems used in some aircraft have a life of only eight to ten hours when operating with a pink IR filter whereas the normal specification of life for such a lamp without the filter is in excess of fifty hours.
2. Present filtering methods leave undesirable signatures in the emitted spectrum above 1 micron wave length. This signature is susceptible to enemy missile homing capabilities and as such produces a missile trap subject to missile homing of early generation sholderfired heat seeking missiles. Efforts to filter out the objectable signature traps still more heat, further reducing the life of the lamp.
3. The high level of trapped heat associated with the IR filtered incandescent lamp provides a forward looking infrared signature for a significant period of time after the lamp is extinguished. Such a hot spot increases the possibility of enemy acquisition of the craft employing such a lamp after the covert lamp has been deenergized.
There is great need for a covert light source to provide IR illumination in the range of 300 meters or less from a helicopter, for example, particularly when operating in a nap of the earth environment.
The present invention is to provide a covert near IR source capable of providing illumination up to 300 meters or so while significantly reducing or eliminating the three problems above enumerated.
It is desirable to extend the reliability of the source to at least 500 hours of operation while eliminating the missile trap signature above 1 micron wave length and to reduce or eliminate the residual infrared signature remaining after the illuminator is extinquished.